Change in Nitric Acid Preservative for Trace Element Samples

Date: Tue, 29 Sep 1998 14:29:30 -0400
From: Nana Frye 
Reply-To: "Nana L Frye, Secretary (OA), Reston, VA "
Organization: U.S. Geological Survey

To: "A  - Division Chief and Staff",
        "B  - Branch Chiefs and Offices",
        "S  - Special Distribution for Research",
        "FO - State, District, Subdistrict and other Field Offices",
        "PO - Project Offices",,
CC: "  WRD Archive File,  "
Subject: OWQ Technical Memo 9806--Change in Nitric Acid Preservative for Trace Element Samples


In Reply Refer To:      September 29, 1998
Mail Stop 412


Subject:        Change in Nitric Acid Preservative for Trace Element Samples


The purpose of this memorandum is to inform Division personnel of 
a change in the concentration, volume, and containers for nitric 
acid used for preservation of water samples for the determination 
of trace elements.


For many years the accepted protocol for the preservation of 
water samples for determination of trace elements has required 
adjustment of sample pH to less than 2 pH units using nitric acid 
(U.S. Environmental Protection Agency, 1979).  The Division has 
provided nitric acid in glass ampoules for this purpose since the 
1970's (Wood, 1976).  The purpose of adjusting sample pH to less 
than 2 is to minimize metal cation precipitation and adsorption 
onto the sample container wall (OWDC, 1982).  However, it has 
been known for some time that nitric acid will also solubilize 
certain elements from glass ampoules, specifically aluminum, 
barium, boron, silica, chromium, and zinc (Open-File Report (OFR) 

As analytical detection limits for trace elements have become 
more sensitive, the use of a higher purity ("ultra-pure" or 
"Ultrex-grade") nitric acid (use of trade, product, or firm names 
is for descriptive purposes only and does not imply endorsement 
by the U.S. Geological Survey) was recommended to reduce possible 
contamination of samples by the preservative (OFR 94-539).  In 
1994, concurrent with the promulgation of the new part-per-
billion sampling protocols, Ultrex-grade nitric acid in Teflon 
vials were made available as an alternative to glass ampoules for 
trace metals preservation.  This change eliminated glass ampoules 
as a source of contamination of field blanks and some 
environmental samples.  Teflon does not contribute detectable 
inorganic contaminants, but the vials are costly and difficult to 
handle in the field.

The Office of Water Quality (OWQ), National Water Quality 
Laboratory (NWQL), and the Quality of Water Service Unit (QWSU) 
have been evaluating polypropylene vials as an alternative to 
both the glass ampoules and Teflon vials for Ultrex-grade nitric 
acid preservative. This option would serve to improve data 
quality at a reasonable cost.  The polypropylene vials are less 
fragile than glass, and the preservative is more easily dispensed 
from them than from glass ampoules or Teflon vials.

Polypropylene is not recommended for storage of concentrated 
nitric acid, but diluted nitric acid is reported to have only a 
minor effect on the material, based on testing by Eagle-Picher 
Industries, Inc. (Rod Neal, oral commun., 1998).  The 
manufacturer of Ultrex nitric acid has agreed to provide the acid 
in a 50-percent concentration (7.5-7.7 Normal) for Division use.  
This diluted form can be stored in polypropylene vials without 
apparent degradation of the plastic for at least one year.  The 
acid will be provided in 2-milliliter quantities so that the 
majority of natural water samples will have a pH below 2 after 
addition of the contents of one vial. Field personnel should be 
aware that unusual matrices may require additional acid to ensure 
a sample pH of <2.0.


Effective October 1, 1998, glass ampoules containing Ultrex 
nitric acid no longer will be available from QWSU.  Districts may 
continue to use Ultrex nitric acid in glass for major-ion 
schedules, and in Teflon vials for trace-element schedules until 
existing stocks are exhausted.  Nitric acid in glass ampoules 
should not be used to preserve samples for trace-element 

In September 1998, Ultrex-grade nitric acid will be available 
from the QWSU in polypropylene vials.  Each vial will contain 2 
milliliters (2 ml) of 50% (by volume) Ultrex-grade nitric acid 
(7.5-7.7 Normal) which is sufficient to preserve the majority of 
natural water samples (8-ounce, -250 ml) at a pH <2,0.  The 
addition of 2 milliliters of the preservative to a sample will 
not introduce any significant dilution error.  Some atypical 
sample sources may require additional acid to ensure a sample pH 
of <2.

Each polypropylene vial will bear a label displaying the name of 
the acid, concentration, volume, lot number, and date of 
preparation.  The preservative will be packaged and shipped in 
boxes of 24 vials, with a Certificate of Analysis from the NWQL.  
The packaging for these vials is approved by the U.S. Department 
of Transportation (DOT), and the vials should be stored and 
transported in the field in the original container.  Empty PP 
vials should be disposed in accordance with local regulations. 
Available information indicates that the polypropylene vials are 
suitable containers for 50 percent nitric acid. However, the OWQ, 
the NWQL, and Ocala QWSU plan to test the vials and acid over 
time to confirm stability. A shelf life cannot be provided at 
this time. However, it is recommended that you buy no more than a 
2-year supply. Until sufficient testing can be done, it is not 
recommended that the vials be stored in field vehicles under high 
heat conditions for extended periods. 

The price of the polypropylene Ultrex nitric acid vials will be 
somewhat more than the glass ampoules but much less than the 
Ultrex acid in Teflon. The price (like other preservatives used 
by the Division) will reflect the costs of the custom-prepared 
acid, polypropylene vials, labels, packaging, and NWQL/QWSU 
quality-assurance and handling.  The vials will be listed in the 
QWSU catalog as: 436FLD Vial (PP), HNO3, 7.5N-7.7N, 2ml, Ultrex. 
Orders may be placed through One-Stop Shopping:, from the QWSU webpage:, or Email to Ultrex 
nitric acid will be available only in the polypropylene vials and 
only in the 2 ml quantity and 7.5-7.7 Normal concentration.

Nitric acid is highly corrosive, even in this diluted form. 
Suitable personal protective gear always should be used when 
handling this chemical (gloves and eye protection).  The Material 
Safety Data Sheet for this chemical is available at:



U.S. Environmental Protection Agency, March, 1979, Methods of 
Chemical Analysis of Water and Wastes.

Wood, W.W., 1976, Guidelines for collection and field analysis of 
ground-water samples for selected unstable constituents: 
Techniques of Water Resources Investigations of the U.S. 
Geological Survey, Book 1, Chap. D2.

Office of Water Data Coordination, 1982, National Handbook of 
Recommended Methods for Water-Data Acquisition, Chapter 5.

Horowitz, A.J., Demas, C.R., Fitzgerald, K.K., Miller, T.L., and 
Rickert, D.A., 1994, U.S. Geological Survey Protocol for the 
Collection and Processing of Surface-Water Samples for the 
Subsequent Determination of Inorganic Constituents in Filtered 
Water, Open-File Report 94-539.

        Janice R. Ward
        Acting Chief, Office of Water Quality

Distribution:   A, B, S, FO, PO
        District Water Quality Specialists
        Regional Water Quality Specialists
        OWQ Staff

This memorandum does not supersede any other Office of Water 
Quality Technical Memorandum.

Key Words: Sample preservation, trace metals, Ultrex, nitric acid

OWQ Technical Memorandum 98.06